1. Route Choice Lecture 11 Norman W. Garrick

2. By Foot, Bus, Tram, Rail, Cable Car
Route Choice
From Kirche Fluntern
To ETH-Hönggerberg
By Foot, Bus, Tram, Rail, Cable Car
What are some factors affecting
route choice?

3. Route Choice or Trip Assignment
Trip assignment is the fourth step of the FOUR STEP process
It is used to determining how much traffic will use each link of the transportation system
Norman W. Garrick

4. Route Choice or Trip Assignment in 4 Step Process
Example
Consider two zones
Hartford Downtown
West Hartford Center
Four Steps
Trip Generation - Determines production from WH Center
Trip Distribution - Gives QIJ - Trips from WH Center attracted to Hartford Downtown
Modal Split - Fraction of QIJ using different modes of travel
Trip Assignment - Which roads? Which bus routes?
Norman W. Garrick

5. Characterizing Road or Transit Network for Trip Assignment
In trip assignment the road or transit network is represented by links and nodes
Links - major roads, including arterials, expressways and freeways (local roads are not usually included - this can be a problem in places like in WH Center were the local road network is very dense and carries a significant portion of the traffic)
Nodes - typically intersections or interchanges, but could be other points that are important to the network
Each node is numbered
Links are specified by the nodes at the end
Each link is associated with an impedance (the impedance might not be the same in each direction
Norman W. Garrick

6. Example Road Network for Trip Assignment1 2 5 5 3 6 7 8 11 4 9 10
1, 2, 3, 4, 5 are zone centroids 12 14 13 5

7. Network B 2 5 1 4 3 (3) (2) (2) (4) (7) (8) (4) (6) (5) (4)
Norman W. Garrick

8. 1 2 3 4 5 Link Array Network B 2 5 1 4 3 (3) (2) (2) (4) (7) (6) (8)
(5)
(4) 3 1 2 3 4 5 J I

9. 1 2 3 4 5 ? Link Array Network B I=1 2 5 1 4 3 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3 1 2 3 4 5 ? J I

10. 1 2 3 4 5 ? Link Array Network B I=1 2 5 1 4 3 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3 1 2 3 4 5 ? J I

11. 1 2 3 4 5 2 5 1 4 3 Link Array Network B I=2 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3 1 2 3 4 5 J I

12. Link Array Network B All Is2
(3)
(2)
(2)
(4)
(7) 5 1
(6)
(8)
(4) 4
(5)
(4) 3 1 2 3 4 5 6 7 8 J I

13. i j wij Link Table Network B 2 1 2 3 5 1 4 3 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3
Norman W. Garrick

14. i j wij Link Table Network B 2 1 2 3 4 5 1 4 3 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3
Norman W. Garrick

15. i j wij Link Table Network B 2 1 2 3 4 5 1 4 3 (3) (2) (2) (4) (7) (6)
(8)
(4) 4
(5)
(4) 3
Norman W. Garrick

16. i j wij Link Table Network B 2 1 2 3 4 5 6 7 8 5 1 4 3 (3) (2) (2) (4)
(7) 5 1
(6)
(8)
(4) 4
(5)
(4) 3
Norman W. Garrick

17. Route Choice Behavior
Trip assignment is based on one of two assumptions about travelers’ behavior
User Equilibrium
System Equilibrium
Based on the assumption that users try to minimize their individual time of travel by going along the shortest path from origin to destination
Based on the assumption that users try to minimize the TOTAL system cost - that is the cost for all users of the system, not just his or her own cost
Route assignment based on user equilibrium requires that we determine the ‘minimum path’ between any two zones or the ‘minimum tree’ which is a diagram showing the minimum path from one zone to all other zones
Norman W. Garrick

18. Network B Determining the Minimum Tree from Node 1
There are two ways to go from
Node 1 to Node 5
All routes to 5 from 1 must pass thru node 4
There are two ways to node 4: and
Route has lower impedance (5 versus 11) 2
(3)
(2)
(2)
(4)
(7) 1 5
(8) 4
(4)
(6)
(5)
(4)
Therefore, route is on the minimum path to 4 (and node 5). To get the minimum tree from 1 eliminate the last link on the longer path. Eliminate link 3-4. 3
Norman W. Garrick

19. Network B Determining the Minimum Tree from Node 42
(3)
(2)
(2)
(4)
(7) 1 5
(8) 4
(4)
(6)
(5)
(4) 3
There is an algorithm for finding the minimum tree
We will not cover the algorithm in this class

20. Network B Tree Table from Node 42
(3)
(2)
(2)
(7)
(4) 1 5
(6) 4
(8)
Node ( j )
Total Impedance to Node j
Node Preceding j 1 2 3 4 5
(4) 3

21. Network B Tree Table from Node 42
(3)
(2)
(2)
(7)
(4) 1 5
(6) 4
(8)
Node ( j )
Total Impedance to Node j
Node Preceding j 1 6 2 3 4 5
(4) 3

22. Network B Tree Table from Node 42
(3)
(2)
(2)
(7)
(4) 1 5
(6) 4
(8)
Node ( j )
Total Impedance to Node j
Node Preceding j 1 6 2 4 3 - 5 7
(4) 3

23. Allocating Traffic to Individual Routes
Once the MINIMUM PATH is determined between different zones then traffic can be allocated to the various links between the zones
One common approach is the FREE FLOW/ALL-OR-NOTHING TRAFFIC ASSIGNMENT Technique
As the name implies, the technique assumes that all traffic between any two zones will use the minimum path between those two zones. The other big assumption is that the minimum path is calculated based on FREE FLOW conditions. In other ways, it is assumed that the minimum path calculations will not be affected by the amount of traffic using that path.
This is obviously an unreasonable assumption. Other traffic assignment techniques have been developed which try to correct for the two big problems with Free Flow/All-or-Nothing Traffic Assignment
Norman W. Garrick

24. Allocating Traffic to Individual Routes (continued)
FREE Flow/Multipath Traffic Technique
Does not assume that all traffic will use the minimum path - instead traffic is assigned to the various paths between the two zones based on their relative impedance. So for example, the path with the minimum impedance will get the most traffic followed by paths with increasing impedance.
This method is still limited by the fact that the impedance is based on free flow assumptions and the impedance value is not changed to reflex the level of traffic loading.
Capacity-Restrained Traffic Assignment Techniques
Accounts for the fact that as the traffic on a link increases, the impedance also increases. Therefore, it is based on an interactive traffic assignment process that re-calculates the impedance to account for the level of traffic assigned to each link. As you can imagine, this is a complex and computer intensive process.
Norman W. Garrick

25. Using Free Flow/All-or-Nothing Assignment - Example
Trip Exchange 1 2 J 1 2 3
Q1j
200
400
800
Q2j
150
100
Q3j
300
600
350 2 2 2 3 10 4 5 6 2 4 4 3 3 3 6 2
Norman W. Garrick

26. Minimum Tree from Zone 1 1 2 2 2 2 3 10 4 5 6 2 3 3 3 6 2
Norman W. Garrick

27. Free Flow/All-or-Nothing Assignment – Zone 1
Trip Exchange 1
1200 2 J 1 2 3
Q1j
200
400
800
Q2j
150
100
Q3j
300
600
350
400
800
400 4 5
400
800 3 6
800
Norman W. Garrick

28. Minimum Tree from Zone 2 1 2 2 2 2 3 4 5 2 4 4 3 3 3 6 2
Norman W. Garrick

29. Minimum Tree – Zone 3 1 2 2 2 2 3 4 5 2 4 4 3 3 3 6 2
Norman W. Garrick